The study area concerns the Akouédo landfill built over an area approximately of 153 hectares. It is located south of the autonomous district of Abidjan (Southern Côte d’Ivoire) and located between latitudes 5˚12' and 5˚45' North and longitudes 3˚42' and 4˚24' West (Figure 1). The Abidjan district includes ten (10)

municipalities to which are added three (3) sub-prefectures: Bingerville, Songon and Anyama. It extends over a total area of 57,735 ha including 8981 ha of lagoon, or 16% of the total area, and 48,754 ha of dry land, or 84% of the total area. According to INS (2021) , this district has a large population estimated at 6,321,017 inhabitants.

The geological context of the city of Abidjan is that of the sedimentary basin of Côte d’Ivoire, 350 km long from East to West and very narrow in width, between 10 and 40 km from North to South. It is affected by a major accident in an E-W direction presenting a route which corresponds substantially to that of the “Ebrié” lagoon. This fault, called the “lagoons fault”, divides the onshore basin into two parts; one slightly thick (around 100 meters) in the North, and the other, very thick (3000 m) in the South. The sedimentary formations are of great variety (Aghui & Biémi, 1984) . We distinguish:

- Quaternary formations which outcrop to the south of the lagoon fault and in the fluvio-lagoon depressions. They are essentially made up of sand, gravelly sand, mud or clay, muddy sand and sandy or silty mud;

- Continental Tertiary formations, which are made up of coarse sands, variegated clays, ferruginous sandstones and iron ores. All these formations are of Mio-Pliocene age and result from the disintegration of the base;

- Formations of the Secondary, Upper Jurassic to Upper Cretaceous and marine Tertiary, are mainly made up of sands, conglomerates, versicolor clays, leafy clays with intercalations of marls and sandstones, sandstones, fluvial sands and sandy limestones sometimes dolomitic. The Paleocene and Eocene are, on the other hand, formed of glauconious clays, sands and small limestone banks.

As for the geology of the Akouédo landfill site, the formations are of Continental Terminal age (Figure 2). They are essentially made up of discontinuous lateritic armor locally covering sandy clays and clayey sands (0 to 70 m); coarse fluvial sands (0 to 90 m), black clays and clayey sands (0 to 10 m). In addition, they are also made up of gravelly sands with variegated clays (0 to 20 m) (Adiaffi, 2008) .

From a hydrogeological point of view, the autonomous district of Abidjan contains continuous type aquifers located in the geological formations of the coastal sedimentary basin (Aghui & Biémi, 1984; Oga, 1998) . These aquifers are composed of aquifers of Quaternary, Tertiary (Mio-Pliocene or Continental Terminal) and Upper Cretaceous (Maestrichtian) age. The depths of the water tables vary from 10 m to 60 m in the South, and 200 m in the North (Blé et al., 2020) .

The study of the Akouédo landfill was possible thanks to equipment consisting of:

- one (01) “SYSCAL JUNIOR” resistivity meter for acquiring electrical data;

- one (01) 12 Volt battery used as power supply;

- electrodes for injecting electric current into the ground and measuring the potential difference;

- four (04) reels of electrical cable for the connections between the resistivity meter and the different electrodes;

- four (04) hammers for installing the electrodes in the ground;

- one (01) compass for identifying the direction of the measurement lines;

- one (01) GPS for recording the coordinates of the electrical sounding points (SE).

Data processing was possible using the following software:

- Ix1D for the interpretation of electrical sounding curves (SE);

- Ipi2win for carrying out geo-electrical sections (2D).

Figure 9 shows a longitudinal section passing through the survey points SE4, SE5, SE8, SE9 and SE10 (see Figure 3). The distribution of subsoil’s resistivities noted the existence of a very conductive body (ρ < 30 Ω·m) enveloped by another relatively conductive ground whose resistivity is between 30 and 60 Ω·m. This conductive body is clearly visible on the cross section passing the boreholes S3, S5 and S6 observed in Figure 10.

The nature of the conductive body corresponds to waste buried at depths ranging from 30 to 40 m. This waste seems to be isolated from the surrounding environment by a ground of variable nature ranging from sandy clay to clayey sand as visible on the model presented in Figure 11. Referring to the drilling data from the installation of piezometers (pz1 and pz2) (see Figure 8), solid waste

would be isolated from groundwater by a small layer of sandy clay. This would not constitute a significant safety barrier against groundwater pollution given that the decomposition of organic waste produces fluid waste (leachate, biogas).

The stratigraphic description of the Akouédo discharge using the electrical resistivity method coupled with that from piezometers (see Figure 5) made it possible to note that around the discharge site, the lithology is as follows:

- Thin clay level, or even non-existent, this discontinuity would highlight the great vulnerability of the aquifer;

- Almost superficial layer made up of sandy clay (or Arab earth sometimes) with a thickness estimated at 3 meters;

- Sandy-clay layer having a thickness of 40 to 50 m. In the field, it is sometimes outcropping in the surroundings of the landfill. In the northern part of the study area, this layer contains shreds of sandy clay indicating lateral variations in lithological facies;

- Land made up of relatively humid sand with an average thickness of 20 m. In certain places, its reduced thickness is probably due to a phenomenon of erosion. This level could reveal the presence of the roof of the underlying crystalline bedrock encountered at approximately 80 m, constituting the bedrock of the area studied (Kouamé et al., 2006; Adiaffi, 2008, Aka, 2013) .

Within the landfill, studies have shown that the waste has an average thickness of 30 m which rests on a thin layer of sandy clay deposited in turn on terrain varying from clayey sand to fine to coarse sand. According to the work of Blé et al. (2020) , the range of water table depths less than 40 m in the southern zone of the Ebrié lagoon is more exposed to rapid contamination if there was pollution on the surface. Thus, the virtual absence of clay soil observed would indicate a high level of vulnerability of the aquifer due to discharge (Soro et al., 2010; Deh et al., 2012) . However, the presence of clay-sand and sand-clay levels, relatively permeable to infiltration of pollutants, would constitute, in addition to sandy levels, an excellent filtering device for leachate in the soil. Likewise, the work of Blé et al. (2020) showed that the waters of the Akouédo area and those of the Nord Riviera well field closer to the discharge are therefore of good quality, because the different physicochemical parameters analyzed respect WHO standards. Thus, the conservation of water quality is linked to the importance of the clay-sand surface layer which behaves like a real filtering layer for this water (Vilomet, 2000) .